The present invention relates to producing thick sheet from direct chill cast aluminum alloy using major cold rolling reductions which are effective to eliminate any furnace annealing step.
Continuous casting of aluminum is well known and taught for example in U.S. Pat. Nos. 5,106,429; 5,329,688; 5,356,495; 5,634,991; and 5,993,573 (McAuliffe et al.; Arvedi et al.; Wyatt-Maer et al.; Kamat; and Selepark et al., respectively). Most of these processes require some sort of an intermediate annealing process. Direct chill casting of aluminum is also well known and taught for example in U.S. Pat. Nos. 4,582,118; 4,610,295; 4,709,740; 4,724,887 (all Jacoby et al.) and 4,282,044 (Robertson et al.), as well as Kent R. Van Horn, Aluminum Vol. 3 xe2x80x9cFabrication and Finishingxe2x80x9d Amer. Soc. For Metals, 1967, pp 18-20 and 40-43; and E. Herrmann et al. Handbook on Continuous Casting, Alcominimum-Verlag, 1980, pp. 1-6. Most of the processes also use various hot rolling and cold rolling processes to produce final sheet, for example, Seidel, in U.S. Pat. No. 6,237,384 B1 provides an additional hot roll between the end of a cooling section and coiling and Sivilotti et al. applies cooling liquid only to the lower surface of continuously moving metal strip.
Of all the steps in fabricating thin or thick metal sheet, the thermal treatment or annealing steps are extremely energy intensive. In today""s energy crisis there is a need to reduce consumption of natural gas or electricity (or other energy sources) needed to heat the metal, during strip fabrication, to a temperature sufficient for stabilizing the metal.
It is one of the main objects of this invention to provide an energy efficient strip fabrication process. It is another object of this invention to eliminate the need for any furnace annealing, thereby saving energy, especially for thicker strip fabrication such as used for truck trailer skins and the like where thickness of from about 1 to 1.5 mm is required.
These and other objects are solved by providing a process for producing thick aluminum sheet consisting of: casting an aluminum alloy ingot; and then cooling and optionally scalping the ingot to provide a smooth surface suitable for hot rolling; and then heating the ingot to from 500xc2x0 C. to 600xc2x0 C. (930xc2x0 F. to 1110xc2x0 F.); and then hot rolling the ingot into a coilable sheet having a thickness of from 3.2 mm to 5.8 mm; and then wrapping the hot sheet into a coil; and then cooling the coiled sheet to less than 60xc2x0 C., preferably; and then cold rolling the cooled sheet from the coil between two opposing rolls to reduce the thickness to from 0.9 mm to 1.5 mm and to provide sheet with a temperature of from 120xc2x0 C. to 160xc2x0 C. (250xc2x0 F.-320xc2x0 F.), where sufficient heat is generated during the cold rolling, to stabilize the aluminum sheet; and then wrapping the reduced sheet into a coil; and then cooling the coiled sheet, preferably, to ambient temperature. This process can use aluminum scrap as a starting material. By xe2x80x9cstabilizingxe2x80x9d is meant low temperature thermal treatment designed to prevent age-softening in certain strain hardened aluminum alloys containing magnesium. Preferably, the ingot is cast by well known direct chill casting.
Additional steps can include tension leveling, that is, a process designed to level, or mechanically flatten, by continuously stretching uniaxially with the assistance of bending, and trimming the edges of the sheet to a desired width and coating the sheet preferably both sides with paint or the like protective material. The heat generated from the tandem cold rolling reductions, preferably two passes, is sufficient to stabilize the aluminum sheet without having to anneal it in a furnace, adding greatly to energy savings. This cold reduction of thick sheet is essential to the process. The sheet produced is thick sheet 0.9 mm to 1.5 mm, useful for example for truck trailer sheet rather than for example cans and the like which require a sheet thickness of less than about 0.5 mm. By truck trailer sheet is meant skin sheet on the trailers of tractor trailers and other van-type mobile carriers as a trailer or truck. The sheet in this process is of such a gauge that the thickness of the hot milled coil and the passes on the cold mill do not require an intermediate anneal.
Other aspects and advantages of the invention will occur to persons skilled in the art from the following detailed description.